Ps 62:1

“Therefore God gave us a pattern of fasting in this deed, so that having a desert, as it were, at the time of fasting we may abstain from feasts, pleasure and women, and so that Eve may not be joined with us so as to subvert us from chaste observance by her alluring persuasion. For he who fasts and is chaste at the time of the 40-day fast seems somehow to dwell in the desert. Clearly, that certain desert is the body of a Christian when it is not filled with food or watered with drinks, but is neglected by the squalor of thirsty starvation. The desert, I say, is our body when the flesh begins to grow weaker by abstinence, when pallor is overcome by thirst, and the unadorned appearance of the whole human being becomes dirty by the contempt of material things. Then Christ the Lord dwells in the desert of our body, when he has discovered our land squalid in hunger and dry by thirst, just as that adage which the prophet David says, "Just like in a deserted land where there are neither roads nor water, so I appeared in your sanctuary." For we are not able to appear to him in the sanctuary in any other way, unless the land of our body has been made desert from worldly delights and without the roads made by devilish desires and without the water from libidinous allurements. Then the Savior, dwelling in this desert of our body, conquers all the factions of the devil and makes our body his own dwelling, safe and secure from the thoughts of this age, so that we may be constituted in solitude within our very selves, as it were, and see nothing but heaven and earth. That is to say, we think about nothing other than the Lord of the heavenly kingdom and the author of the earthly resurrection.”

“"God, my God, unto You from the light I watch" [Psalm 63:1]. What is to watch? It is, not to sleep. What is to sleep? There is a sleep of the soul; there is a sleep of the body. Sleep of body we all ought to have: because if sleep of body is not taken, a man faints, the body itself faints. For our frail body cannot long sustain a soul watching and on the stretch on active works; if for a long time the soul shall have been intent on active pursuits, the body being frail and earthly holds her not, sustains her not for ever in activity, and faints and falls. Therefore God has granted sleep to the body, whereby are recruited the members of the body, in order that they may be able to sustain the soul watching. But of this let us take heed, namely, that our soul herself sleep not: for evil is the sleep of the soul. Good is the sleep of the body, whereby is recruited the health of the body. But the sleep of the soul is to forget her God. Whatsoever soul shall have forgotten her God, sleeps. Therefore the Apostle says to certain persons that forgot their God, and being as it were in sleep, did act the follies of the worship of idols— the Apostle, I say, says to certain persons, "Rise, you that sleepest, and rise up from the dead, and Christ shall enlighten you." [Ephesians 5:14] Was the Apostle waking up one sleeping in body? Nay, but he was waking a soul sleeping, inasmuch as he was waking her, in order that she might be lightened by Christ. Therefore as to these same watchings says this man, "God, my God, unto You from the light I watch." For you would not watch of yourself, unless there should arise your Light, to wake you from sleep. For Christ lightens souls, and makes them to watch: but if His light He takes away, they slumber. For for this cause to Him there is said in another psalm, "Lighten my eyes, that I may never slumber in death.". ..”